An example of a spark plug conventionally used for an internal combustion engine is a spark plug in which spark discharge is produced in a gap (spark discharge gap) between a columnar center electrode and a bent ground electrode. In such a spark plug, a ceramic insulator is disposed between the center electrode and a metallic shell on which the ground electrode is provided. If the ceramic insulator has a defect such as a pinhole, discharge may occur through the defect, and a through hole may be formed in the ceramic insulator to extend in the thickness direction thereof. If such a through hole is formed, there arises a possibility of failure to produce normal spark discharge. In view of the foregoing, during a process of manufacturing a spark plug, there has been performed a step (hereinafter referred to as a withstanding voltage test step) of applying a high voltage between an electrode (e.g., a center electrode) disposed in an axial hole of a ceramic insulator and an electrode (e.g., a metallic shell) disposed outside the ceramic insulator and determining whether or not the ceramic insulator has a defect, on the basis of a voltage waveform measured at that time or an image captured at that time. In the withstanding voltage test step of Patent Document 1, the entire ceramic insulator is disposed in a closed container, and a high voltage is applied between the electrode disposed in the axial hole of the ceramic insulator and the electrode disposed outside the axial hole in a state in which the interior of the closed container is pressurized.
However, in the spark plug production method of Japanese Patent Application Laid-Open (kokai) No. 2007-134132, since the entire ceramic insulator is disposed in the container, the container must have a very large volume. Therefore, a long time is needed to pressurize and depressurize the interior of the container, and a long time is needed to dispose the ceramic insulator into the container and remove it from the container. Therefore, there have been a problem of requiring a longer time to produce the spark plug and a problem of lower production efficiency. In addition, since the container for accommodating the ceramic insulator is large in size, there has been a problem that the size of the entire production device increases. Also, since the surface area of the container is large, there has been a problem that a material of high rigidity must be used as the material of the container in order to allow the container to have a sufficient degree of robustness such that the container can withstand the application of pressure of, for example, about 2 MPa (mega pascal), and therefore, the container has a very large weight. Further, since it is necessary to provide a lock mechanism or a like mechanism at an opening of the closed container through which the ceramic insulator is disposed in the container and removed therefrom, and to provide means for supplying electric power to the interior of the closed container, there has been a problem that production cost increases.
Further, in the spark plug production method of Japanese Patent Application Laid-Open (kokai) No. 2007-134132, the ceramic insulator solely undergoes the withstanding voltage test, and the ceramic insulator is not tested in a state in which the ceramic insulator is actually used; i.e., in the form of an assembly in which the ceramic insulator is combined with a metallic shell. Therefore, there has been a problem of failing to meet the market's need to produce a spark plug while performing the withstanding voltage test in the configuration in which the insulator is actually used.
The present invention has been accomplished to address the above-described problem. The present invention can be realized as the following modes or application examples.